1. Field of the Invention
The present invention relates in general to a method of calibrating a touch screen used in conjunction with a display such as a cathode ray tube (CRT); and more particularly, to a method for automatically calibrating a CRT touch screen prior to each session during which the touch screen is used.
2. Description of the Related Art
The use of a touch screen overlaying a CRT screen is well known for inputting data into a computer. Such touch screens are substantially transparent and may include a net of wires or conductive plastic or utilize ultrasonic or photo-detectors at the edges of the screen. Since the purpose of such touch screens is to detect the location of a finger or other object disposed in proximity to an area on the CRT display, it is necessary for the computer receiving data from a touch screen to know the correspondence between the display and the touch screen. Therefore, it is necessary to calibrate the coordinates output by the touch screen with the coordinates of points onthe display.
A conventional method for establishing the correspondence between a display and a touch screen is described in U.S. Pat. No. 4,442,317 to Jandrell. Two techniques are described in Jandrell. The first technique utilizes a pair of scaling and offset amplifiers in each of the horizontal and vertical directions to adjust the output of the touch screen to match that of the display. The second technique is used in the case of non-uniform electric fields and is based on mapping between display positions and probe potentials. In both cases, it is necessary for manual adjustments to be performed, either to the scaling and offset amplifiers or to establish the mapping between the touch screen and the displayed data.
After a touch screen has been calibrated as described in Jandrell, the calibration operator can be expected to properly input data using the touch screen. However, studies have been performed which indicate that different people tend to touch the screen of a display at different locations when selecting the same area on the CRT. For example, someone seated below the screen may consistently touch the bottom of a displayed symbol, while someone standing above the screen may consistently touch a higher point. Similarly, one person may touch to the left and another to the right of a symbol. Therefore, it is desirable to make some allowance for individual variation in interaction with a touch screen overlaying a computer display.
One solution to the problem of individual variance is described in U.S. Pat. No. 4,396,977 to Slater et al. The solution in Slater et al. is to provide feedback to an operator to indicate the position detected as having previously been touched. A two step process is used in the case of critical inputs in which the first step involves selecting a command by touching one of the options displayed on the screen and the second step involves touching another area on the screen to execute the command selected. This method requires that the feedback routines and two step selection process be used in every program utilizing the touch screen input. In addition, there is no allowance for a severely misaligned touch screen which would require an operator to remember not to touch a desired symbol but to touch the next symbol to the right, for example. In other words, this method results in the training of an operator rather than the "training" or calibration of the equipment.